1. Field of the Invention
This invention relates generally to data disk drive systems and more specifically to servo patterns for flexible data disks of such systems.
2. Description of the Prior Art
In electronic computer technology, it is common to store data in binary form on the face of a rotatable disk. The face of the disk is coated with a magnetizable substance such as iron oxide. The disks are operated by rotating them like phonograph records and the binary data is encoded upon, or retrieved from, the face of the disk by a movable magnetic transducer device called a read/write head or transducer head. The binary information is encoded on the face of the disk in concentric rings, called tracks, and the read/write head can move radially along the disk face to select a particular track to record or retrieve information. The data disks can be rigid or flexible.
The rigid disks typically have data densities of about five hundred to twelve hundred tracks per inch of radius of the disk. The flexible disks typically have densities of forty-eight or ninety-six tracks per inch. Because of the high density, precise positioning of the read/write head is necessary so that the head can accurately gain access to a particular desired track on the surface of the disk.
In order to obtain precise positioning, most disk drives use open loop stepper motors. A few disk drives contain a glass scale which provides the disk drive with information on the position of the read/write head. Rigid disk systems sometimes use a different method to obtain positioning information. These rigid disks have a servo sector which is read once per revolution. The servo sector tracks contain positioning data to help the transducer heads stay on the data track. The servo tracks are radially off-set from the data tracks such that a transducer head passes between the servo tracks when it is positioned along a data track.
The consecutive servo tracks alternate between having an "A" burst and a "B" burst. The transducer head reads the "A" and "B" bursts from the servo tracks on either side. The intensities of the "A" and "B" bursts are measured and the head is adjusted to keep the head midway between the servo tracks and thus directly on the data track.
In order for the head to read the servo bursts, the disk drive system must know when to look for them. In order to do this, rigid disks have synchronization marks (sync marks) at the beginning of each servo frame or sector. A servo frame is considered to be a unit of servo information. Some dedicated servos have up to eight thousand such frames per track. When the head reads a synchronization mark, the disk drive system can generate timing pulse windows to look for the servo bursts. The transducer head reads the two servo tracks on either side of it at the same time. The magnetic pulses which comprise the synchronization marks on the consecutive servo tracks must line up perfectly or they will cancel each other out and the disk drive system will not be able to recognize them as the synchronization marks which signify the beginning of a servo sector.
In rigid disk systems, the radial alignment of the synchronization marks on consecutive servo tracks is not a problem. Rigid disks typically have clocking information located on one surface of the disk. The clocking information is read from one side of the disk as the servo tracks are being written on the other side of the disk. Exact alignment of the sync marks during encoding of the disk is thus possible.
In the past, flexible disk systems have not used servo sectors for positioning. The flexible disks are difficult to encode with the servo pattern. When using flexible disks, one side of the disk cannot be read at the same time that the other side of the disk is being encoded. The thin surface of the flexible disk allows cross-talk between the heads on either side. Thus, the clocking technique used to encode servo information on hard disks is not applicable to encoding servo information on flexible disks. Using traditional methods to encode flexible disks results in the synchronization marks being radially misaligned from track-to-track. The misaligned synchronization marks cancel each other out when the transducer head passes between them. When this happens, the disk drive system is unable to generate the timing windows necessary to read the "A" and "B" bursts.